The research in progress seeks to take advantage of mutants in the nematode Caenorhabditis elegans that have abnormal muscle structure to probe basic problems in biology. For instance, how do genes specify the three dimensional structure of muscle in both time and space? How do muscle proteins interact to produce contraction? Mutations in the myosin heavy chain gene unc-54 are being studied intensively to learn about the genetic organization of that gene, and the function of its product, myosin heavy chain, in muscle contraction. Reversion analysis of mutants has uncovered suppressors of various kinds, and these are being developed as additional genetic tools. One suppressor of unc-54 mutations, sup-3, may involve a second myosin gene, causing increased levels of its product to compensate for mutations in the first gene. Other genes are being studied to determine what their product is. By gaining a detailed understanding of the genetic specification of muscle in C. elegans, general principles may be found that underlie muscle structure, function and development, common to all animals.